RUST-1414 disable causal consistency for implicit sessions (#969)

Author: abr-egn

src/client/executor.rs

@@ -495,10 +495,7 @@ impl Client {
                 }
                 // If this is a causally consistent session, set `readConcern.afterClusterTime`.
                 // Causal consistency defaults to true, unless snapshot is true.
-                else if session
-                    .options()
-                    .and_then(|opts| opts.causal_consistency)
-                    .unwrap_or(true)
+                else if session.causal_consistency()
                     && matches!(
                         session.transaction.state,
                         TransactionState::None | TransactionState::Starting

src/client/session.rs

@@ -281,6 +281,12 @@ impl ClientSession {
         self.operation_time
     }
 
+    pub(crate) fn causal_consistency(&self) -> bool {
+        self.options()
+            .and_then(|opts| opts.causal_consistency)
+            .unwrap_or(!self.is_implicit())
+    }
+
     /// Mark this session (and the underlying server session) as dirty.
     pub(crate) fn mark_dirty(&mut self) {
         self.server_session.dirty = true;

src/test/spec/sessions.rs

@@ -27,8 +27,7 @@ use crate::{
 #[cfg_attr(feature = "tokio-runtime", tokio::test(flavor = "multi_thread"))]
 #[cfg_attr(feature = "async-std-runtime", async_std::test)]
 async fn run_unified() {
-    // TODO RUST-1414: unskip this file
-    let mut skipped_files = vec!["implicit-sessions-default-causal-consistency.json"];
+    let mut skipped_files = vec![];
     let client = TestClient::new().await;
     if client.is_sharded() && client.server_version_gte(7, 0) {
         // TODO RUST-1666: unskip this file